1. Field of the Invention
The invention relates generally to a device structure for up and down conversion of radiation, typically solar, wherein high energy wavelengths are down converted to lower energy and low energy wavelengths are up converted to higher energy to improve adsorption by a photovoltaic device.
2. Description of Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98
As an alternative approach to multiple junction solar cells where specific materials are matched to discrete portions of the solar spectrum, conversion works on the principle of moving parts of the spectrum to the wavelength band of a single junction cell. For example it is widely accepted that a single junction, single crystal silicon solar cell has an optimum performance in the wavelength range 500 to 1,100 nm, whilst the solar spectrum extends from 400 nm to in excess of 2500 nm.
Rare earths, the lanthanide series, have long been known for the unique optical properties in which the incomplete, 4f shells exhibit multiple optical transitions many of which lie within the solar spectrum. An example of some of these optical transitions are:
Er: 410, 519, 650, 810, 972, 1,529 nm
Yb: 980 nm
Tb: 485 nm
As used herein [RE1, RE2, . . . RE10] are chosen from the lanthanide series of rare earths from the periodic table of elements consisting of {57La, 58Ce, 59Pr, 60Nd, 61Pm, 62Sm, 63Eu, 64Gd, 65Tb, 66Dy, 67Ho, 68Er, 69Tm, 70Yb and 71Lu} plus yttrium, 39Y, and scandium, 21Sc, are included as well for the invention disclosed.
A more complete list can be found in the technical literature. In addition certain of these rare earths, sometimes in combination with one or more rare earths, can absorb light at one wavelength (energy) and re-emit at another (energy). This is the essence of conversion; when the incident energy per photon is less than the emission energy per photon the process is referred to as up conversion. Down conversion is the process in which the incident energy per photon is higher than the emission energy per photon. An example of up conversion is Er absorbing at 1,480 nm and exhibiting photoluminescence at 980 nm.
One concept was patented in U.S. Pat. No. 3,929,510; more recent work in this field has mainly focused on the addition of rare earths to phosphorescent compounds. The historical approaches however add a conversion layer to either a completed solar cell or module with the majority also requiring an additional reflective component to return the converted spectrum back into the cell so that it can contribute to the generated photocurrent. U.S. Pat. No. 7,184,203 discloses up and down conversion with a rare earth compound comprising a rare earth element and at least one other element selected from chalcogens, halogens, nitrogen, phosphorus and carbon; wherein the rare earth compound is not mixed with compounds containing other rare earth elements and wherein the rare earth compound is irradiated at a sufficient intensity to heat the rare earth compound to facilitate electronic transitions. U.S. Pat. No. 7,184,203 does not teach or suggest using a rare earth compound in conjunction with a photovoltaic device; U.S. Pat. No. 7,184,203 teaches away from the use of a rare earth compound with relatively low intensity radiation at room temperature for up or down conversion.